Driving apparatus for motor vehicle

ABSTRACT

A driving apparatus for a motor vehicle, which employs a power transmission mechanism for transmitting the rotation of a driving motor of the motor vehicle to the driving wheels through a transmission gear mechanism, and in which in order to operate said transmission gear mechanism to a change gear ratio of large output torque at an intersection point between the revolutional speed-revolutional torque curves of an output shaft of said transmission gear mechanism as is obtained at optional change gear ratios, the revolutional speed corresponding to said intersection point between said revolutional speed-rotational torque curves is detected to automatically operate said transmission gear mechanism.

tlnited States Patent Sugiura, deceased et al.

[ Feb. 26, 1974 DRIVING APPARATUS FOR MOTOR 3,604,288 9/1971 Mori 74/752D x VEHICLE 3,657,934 4/l972 Ito et al. 74/866 X Inventors: i gg L g ffPrimary Examiner-Arthur T. McKeon At ,A r, F'-C dAt l admmistratnx',Takeshl Monoka; tome), gen or ralg an n one] I -R5221 ;Kasama, both ofKatsuta, 57] ABSTRACT A driving apparatus for a motor vehicle, which em-[73] Asslgnee= d" Tokyo Japan ploys a power transmission mechanism fortransmit- [22 Filed; 1 1971 ting the rotation of a driving motor of themotor vehicle to the driving wheels through a transmission gear [21]Appl l092 mechanism, and in which in order to operate said transmissiongear mechanism to a change gear ratio of 52 us. Cl 180/65 R, 74/752 D,74/866 large Output torque at an intersection Point between [51] [m C]360k 1/00, F16h 3/74 B601 21/00 the revolutional speed-revolutionaltorque curves of 5 Field f Search 74 752 13 3 7 5; an output shaft ofsaid transmission gear mechanism 290/7 as is obtained at optional changegear ratios, the revolutional speed corresponding to said intersectionpoint [56] Referen e Cit d between said revolutional speed-rotationaltorque UNITED STATES PATENTS curves is detected to automatically operatesaid transmission gear mechanism. 3,628,62l 12/1971 Lee .4 /65 R3,667,325 6/1972 Ito et al. 74/866 6 Claims, 4 Drawing Figures 0.0SOURCE CO/WPOLLER e 0. c 7'/?AN$M/$S/0N L WHEELS MOTOR L GEAR SPEEDCHANGE OPERATOR 5, 550 CHANGE D/RECTOR -8 SPEED DETECTOR PATENTED3,794,133

' SHEET 1 BF 4 S0uPcE CONTROLLER 5 6 0.6 rPA/vSM/SSm/v MOTOR @4 GEAR g;WHEELS SPEED CHANGE oPEPA roP SPEED CHANGE a/PEcroP -6 SPEED oErEcroPINVENTOR5 TAKESHI MORIOKA RYOJI KAEJAMA v SHINZO SUGIURA Q- L O tQHLQQ;HLQQ ATTORNEYS PATENTEDFEBZBIQH $794,133

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SHEU l} 0F 4 PUL SE GENE/B470? INVENTORS TAKESHl MoRloKA RYOSl KAsAMA,

SHINZO sueiuRA BY Whamm /Q; 4r H-LQIQ ATTORNEYfi DRIVING APPARATUS FORMOTOR VEHICLE BACKGROUND OF THE INVENTION 1. Field of thelnvention Thepresent invention relates to driving apparatus in motor vehicles, andmore particularly it intends to suggest a mechanism for automaticallyoperating the change gear ratio of a transmission gear mechanisminterposed between a driving motor and driving wheels.

2. Description of the Prior Art In apparatus for driving a vehicle withan electric motor employed as a source of power, the driving motor isadvantageously made as a low current type in order to make a controlcircuit small in capacity. When the motor is therefore made as a lowcurrent type, it should be made large in configuration or therevolutional speed should be decreased by means of a reduction gearmechanism to increase the revolutional torque in order to obtain a largerevolutional torque. Since, with the reduction gear mechanism, the motoris over-rotated in the case of high-speed running of the vehicle, it isrequired that the reduction gear mechanism be operated to change thechange gear ratio. The motor vehicle, however, has a great advantage inthat it may electrically control the revolutional speed of the motorover a wide range, thereby to realize a speed control using no gearmechanism for the speed change operation. Accordingly, it leads todeprivation of this advantage and it is not preferable to use a speedchange mechanism of manual operation as in conventional vehicles drivenby an internalcombustion engine.

SUMMARY OF THE INVENTION An object of the present invention is toprovide a driving apparatus which may accomplish preferable drivingcharacterwstics over a wide range using an electric motor of small-sizedand high-speed type.

Another object of the present invention is to provide an easilymanipulated driving apparatus for a motor vehicle, by automating theoperation of a transmission gear mechanism interposed between a drivingmotor and the driving wheels.

Still another object of the present invention is to provide an automaticspeed-change mechanism for effectively transmitting the revolutionaltorque characteristics of a driving motor to the driving wheels.

Further objects of the present invention will be understood from thedescription of typical embodiments.

A feature of the present invention resides in that a driving motordesigned in a small-sized and high-speed type is coupled through atransmission gear mechanism to a driving shaft, and that the change gearratio of the transmission gear mechanism is automatically changedover toone increasing the revolutional torque, at an intersection point betweenthe revolutional speedrevolutional torque characteristic curves of thedriving shaft as are obtained under different change gear ratios inpredetermined running regions of the driving motor.

BRIEF DESCRIPTION OF THE DRAWING FIG. I is a block diagram of the wholedrive system of the present invention.

FIG. 2 is a graph of output characteristic curves of an output shaft ofa transmission gear mechanism.

FIG. 3 is an electrical connection diagram of a system for controlling amotor.

FIG. 4 is an electrical connection diagram ofa system for automaticallycontrolling the transmission gear mechanism.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The whole drive system will bedescribed with reference to FIG. 1. Numeral l designates a series-woundDC motor for drive, which is electrically connected through a current(voltage) controller 3 to a DC power source 2 and which is operated byelectric power supplied from the DC power source 2. An output shaft 4 ofthe motor 1 is coupled through a transmission gear mechanism 5 of thetype disclosed in U.S. Pat. No. 3,448,640 to a driving axle 6. Thevehicle driving axle 6 serves to rotate driving wheels 10 to driveforward the vehicle. The transmission gear mechanism 5 is operated by aspeed change operator 7 (by means of electromagnetic force, hydraulicpressure, or the like) to have the transmission gear ratio changed. Thespeed change operation is effected such that the speed of revolution ofthe driving axle 6 is detected by a revolutional-speed detector 8 of thetype disclosed in U.S. Pat. No. 3,448,640, and that a gear ratio isindicated in conformity with a predetermined pattern by a speed changedirector 9. In actuality, where the speed of revolution is lower than apredetermined value, the speed change operation is carried out so as tomake the transmission gear ratio (the reduction gear ratio) large.

In accordance with the objects of the present invention, there is usedas the electric motor 1 one of smallsize and high-speed type (in whichalthough the maximum torque is small, decrease in the revolutionaltorque is comparatively small even at high speed).

Referring to FIG. 2, the speed change operation pattern will beexplained. The efficiency of the transmission gear mechansm 5 is assumedto be percent. The revolutional speed-revolutional torque characteristiccurve of the motor 1 is set at curve I in FIG. 2, in which curve I isfor a transmission gear ratio of 1 while curve II is for a transmissiongear ratio of 2 (decelerating the speed to one-half). Curve III depictsthe rotational torque T characteristic versus load current I.

When a certain fixed terminal voltage is supplied to the driving motor 1and the change gear ratio of the transmission gear mechanism 5 is made1, the revolutional torque T characteristic versus the revolutionalspeed N of the driving axle 6 becomes curve I in FIG. 2. Thecharacteristic has a feature that, while the revolutional torque T in alow-speed region is small, the revolutional torque T in a high-speedregion is comparatively large. In contrast, curve II shows a case wherethe motor 1 is operated in the same state with the change gear ratio ofthe transmission gear mechanism 5 being made 2, and is a characteristicbeing one half in the revolutional speed N and double in the revolutional torque T. Then, since the revolutional torque T which may betaken out from the driving axle 6 should advantageously be larger, it ismore advantageous to use the driving system such that, in the higherrevolutional-speed region with respect to a revolutional speed NPcorresponding to an intersection point P between the curves I and II,the change gear ratio of the transmission gear mechanism 5 is made 1,while in the lower revolutional-speed region, that of the same is made2. More specifically, assuming that the maximum current which may becaused to flow to the driving motor 1 be I,,,,,, if the change-gearratio of the transmission gearings is 1, the maximum revolutional torqueT,,,,,, is obtained along curve I from the driving axle 6. When,however, the change gear ratio of the transmission gear mechanism 5 ismade 2 at the revolutional speed NP corresponding to the intersectionpoint between the curves I and II, the revolutional speed of the motor 1is doubled, but the revolutional torque T of the driving axle 6 mayreach a revolutional torque of 2T,,,,,, along curve II. In a state inwhich the driving axle 6 is in the region above the revolutional speedNP, if the change gear ratio of the transmission gearings 5 is made 1,the revolutional torque T of the driving motor 1 appears at the drivingaxle 6 as it is, and a high revolutional torque may be maintained up tothe maximum allowable speed of revolution N of the driving motor 1.

Accordingly, when the revolutional speed NP of the driving axle 6 ascorresponds to the intersection point P between the curves I and II isdetected to automatically operate the transmission gearings 5, thedriving axle 6 may output large revolutional torques is a widerevolutional-speed region.

Referring now to FIG. 3, description will be made of an embodiment ofelectrical means for controlling the driving motor 1. Shown at 20 arestorage batteries, which are connected through a thyristor choppercircuit 21 to the series-wound DC motor 1. The thyristor chopper circuit21 is constituted by a main thyristor 22 connected in forward polaritieswith respect to a load current, a by-pass diode 23 connected in theopposite direction to and in parallel with the main thyristor 22, aseries circuit for commutation consisting of a capacitor 25 and areactor 26, which circuit is connected through a reverse-currentblocking diode 24 to the main thyristor 22 in parallel therewith, anauxiliary thyristor 27 for discharging charges of the capacitor 25 inthe series circuit to the reactor 26, thereby providing an oscillatingcurrent to turn off the main thyristor 22, and a resistor 28 forcharging the capacitor 25. Such main current circuit operates asfollows:

First of all, the capacitor 25 is charged in illustrated polarities bythe storage batteries 20 through a circuit of the reactor 26 diode 24electric motor 1 and the resistor 28. Upon subsequently turning on themain thyristor 22, the voltage of the storage batteries 20 is applied tothe motor 1, and the motor 1 starts revolution. When the auxiliarythyristor 27 is subsequently turned on after a time T,, charges in thecapacitor 25 are discharged to the reactor 26. Thereafter, the capacitor25 is charged in the opposite polarities to those as shown byself-induction of the reactor 26. Since the voltage of the capacitor 25is thereafter applied through the diode 24 to the main thyristor 22 inbackward polarities, the main thyristor 22 is forcibly turned off.Simultaneously therewith, the voltage is also applied to the auxiliarythyristor 27 in backward polarities to also turn off the auxiliarythyristor 27. Thereafter, the charges of the capacitor 25 are recoveredthrough the diodes 23 and 24 to the capacitor 25 in the illustratedpolarities. Of course, the recovering process also According to suchchopper control, the average voltage EI(V) applied to the motor 1becomes, when the 1 terminal voltage of the storage batteries 20 isexpressed utilizes the self-inductance action of the reactor 26.

by 520w E1 E20 (T /T, T

(VI It is accordingly made possible to control the motor 1 by varyingeither or both of the time widths T and T The embodiment shown in FIG. 3makes the time width T, T constant, and controls the ratios. Hereinbelowwill be described control means therefor.

While numeral 29 indicates auxiliary storage batteries, it is alsopossible, if necessary, to voltage-regulate the storage batteries 20 toreplace them. Lead wires 30 and 31 are led out from electrodes of thebatteries 29. Shown at 32 is an astable oscillator circuit of fixedfrequency which uses a unijunction transistor 33, a resistor 34 and acapacitor 35. One of the bases of the unijunction transistor 33 isconnected through a resistor 36 to the lead wire 30, while the otherbase is connected through the primary coil 38 of a pulse transformer 37to the lead wire 31. The resistor 34 and the capacitor 35 are connectedin series, the intermediate connection point is connected to the emitterof the unijunction transistor 33, an outer end of the resistor 34 isconnected to the lead wire 30, and an outer end of the capacitor 35 isconnected to the lead wire 31. The first secondary coil 39 of the pulsetransformer 37 is connected through a diode 40 across the gate andcathode terminals of the main thyristor 22 in the thyristor choppercircuit 21. Shown at 41 is a monostable multivibrator including NPN-typetransistors 42 and 43, in which the emitters of both the transistors arecommonly connected to the lead wire 31, while the collectors arerespectively connected through resistors 44 and 45 to the lead wire 30.The base of the transistor 43 being normally in the conducting state isconnected through a variable resistor 46 to the lead wire 30, and isconnected through a capacitor 47 to the collector of the transistor 42on the other side. The variable resistor 46 and the capacitor 47 serveto control the metastable period of time of the monostablemultivibrator, and the former 46 is adapted to be operated by anaccelerator pedal. The base of the transistor 42 being normally in thenon-conducting state is connected to the collector of the transistor 43through a parallel circuit consisting of a resistor 48 and a capacitor49, and is connected to the lead wire 31 through a resistor 50. Thesecond secondary coil 51 of the pulse transformer 37 is connectedthrough a diode 52 to the base and emitter electrodes of the transistor42, and is so arranged as to drive the monostable multivibrator 41 intothe metastable state. Numeral 53 indicates a PN P-type transistor foramplification, the emitter of which is connected to the lead wire 30,the base is connected through a resistor 54 to the collector of thetransistor 43, and the collector is connected through a primary coil 56of a pulse transformer to the lead wire 31. A secondary coil 57 of thepulse transformer 55 is connected through a diode 58 across the gate andcathode of the auxiliary thyristor 27 in the thyristor chopper circuit21.

With the above construction, the capacitor 35 in the astable oscillatorcircuit 32 is charged through the resistor 34. Every time the terminalvoltage of the capacitor reaches a predetermined emitter voltage of theunijunction transistor 33, the charges are discharged to the primarycoil 38 of the pulse transformer 37, to generate pulse voltages in thesecondary coils 39 and 51. The circuit constants are set such that thegeneration interval of the pulse voltages is made constant at the timewidth T T When the pulse voltage is induced in the secondary coil 39;,it turns on the main thyristor 22 in the thyristor chopper circuit 21through the diode 40, thus to feed the motor 1 with the supply voltage.Simultaneously, the voltage is also induced in the secondary coil 51,and it turns on the transistor 42 in the monstable multivibrator 41 tobring it into the metastable state. The metastable period of time of themonostable multivibrator 41 is determined by the time constant betweenthe resistor 46 and the capacitor 47, and after the lapse of this time(assumed to be T the transistor 43 turns on to return to the stablestate. Then the transistor 53 also turns on to cause a current to flowthrough the primary coil 56 of the pulse transformer 55, so that theauxiliary thyristor 27 in the thyristor in the secondary coil 57. It isas stated previously that the thyristor chopper circuit 21 is turned offby the firing of the auxiliary thyristor 27.

Accordingly, if the resistance 46 is varied by the accelerator pedal soas to change the metastable time T, of the monostable multivibrator 41,the average output voltage of the thyristor chopper ciruit 21 may becontrolled.

With reference to FIG. 4, there will now be described a control systemfor the speed change operation of the transmission gear mechanism 5.Lead wires 30 and 31 are extensions of those shown in FIG. 3. First,NPN- type transistors 60 and 61 are used to constitute a monostablemultivibrator 62. The emitters of the transistors 60 and 61 areconnected to the lead wire 31, the collectors are respectively connectedthrough resistors 63 and 64 to the lead wire 30, and the base of thetransistor 61 is connected through a resistor 65 to the lead wire 30 andis connected through a capacitor 66 to the collector of the transistor60. The base of the transistor 60 is connected to the collector of thetransistor 61 through a parallel circuit consisting ofa resistor 67 anda capacitor 68, and is connected to the lead wire 31 through a resistor69. Shown at 70 is apulse generator, which generates pulse voltages inresponse to revolution of the driving axle 6 and which is adapted toturn on the transistor 60 through a diode 71. Numeral 72 indicates anaveraging circuit, which is constructed such that the collector voltageof the transistor 61 in the monostable multivibrator 62 is taken outthrough a diode 73 and a resistor 74 into a smoothing condenser 75 andthat it is applied through voltage divider resistors 76 and 77 to thebase of an input transistor 79 of a Schmitt circuit 78. The Schmittcircuit 78 serves to detect a voltage corresponding to the revolutionalspeed NP at the intersection point P illustrated in FIG. 2. NPN-typetransistors 79 and 80 have the emitters connected in common, andthereafter connected through a resistor 81 to the lead wire 31. Thecollector of the transistor 79 is connected through a resistor 82 to thelead wire 30. The transistor 80 has the collector connected through aresistor 83 to the lead wire 30, and connected through a feedbackresistor 84 to the base of the transistor 79, so as to provide ahysteresis characteristic. The base of the transistor 80 is, on onehand, connected through a resistor 85 and a capacitor 86 to thecollector of the transistor 79 at the preceding stage, while it is', onthe other hand, connected through a resistor 87 to the lead wire 31.Numeral 88 indicates a PNP-type amplifier transistor, the emitter ofwhich is connected to the lead wire 30, and the base is connectedthrough a resistor 89 to the collector of the transistor 89 and througha resistor 90 to the lead wire 30. The collector of the transistor 88 isconnected through an electromagnetic coil 91 to the lead wire 31. Theelectromagnetic coil 91 is for the speed change operation of thetransmission gear mechanism 5, and operates to make the transmissiongear ratio 2 (to decrease the speed to one half) when a current flowstherethrough. Shown at 92 is an NPN-type amplifier transistor, theemitter of which is connected to the lead wire 31, while the base isconnected through a resistor 93 to the collector of the transistor 80and through a resistor 94 to the lead wire 31. The collector isconnected through an electromagnetic coil 95 to the lead wire 30. Theelectromagnetic coil 95 effects the speed change operation for thetransmission gear mechanism 5, and functions to make the change gearratio 1 (to directly couple the driving axle to the driving motor) whena current flows therethrough.

In the above construction, the pulse generator generates pulse voltagesat intervals proportional to the revolutional speed N of the drivingaxle 6, to bring the monostable multivibrator 62 into the metastablestate. More specifically, an output pulse of the pulse generator 70turns on the transistor 60 of the monostable multivibrator 62, and turnsoff the transistor 61. When the transistor 61 is turned off, thecollector voltage becomes the same as the electric potential of the leadwire 30 thereby charging the capacitor through the diode 73 and theresistor 74. Since the non-conducting time width of the trnasistor 61 isdetermined to a fixed value by the time constant between the resistor 65and the capacitor 66, the capacitor 75 is charged by that number ofrectangular wave voltages which corresponds to the revolutional speed Nof the driving axle 6. The terminal voltage ofthe capacitor 75 averagedby this capacitor becomes one of a value proportional to therevolutional speed N.

Therefore, if the input voltage of the Schmitt circuit 78 as appliedafter the voltage division by the resistors 76 and 77 is lower than aset voltage of the circuit (a voltage corresponding to the revolutionalspeed N is made the set voltage), the transistor 79 is turned off whilethe transistor is turned on, so that the transistor 88 is turned on tocause the current to flow through the electromagnetic coil 91. Aspreviously stated, the electromagnetic coil 91 makes 2 the change gearratio of the transmission gearings 5 when the current flowstherethrough. As a result, the driving axle 6 is operated in this regionunder the characteristic shown at curve 11 in FIG. 2.

Next, when the revolutional speed N of the driving axle 6 rises and theinput voltage of the Schmitt circuit 78 reaches the voltagecorresponding to the intersection point P, the transistor 79 is turnedon while the transistor 80 is turned off. Accordingly, the transistor 88is turned off to cut off the current of the electromagnetic coil 91, andinstead, the transistor 92 is turned on to cause the current to flowthrough the electromagnetic coil 95. As stated previously, when thecurrent flows through the electromagnetic coil 95, the change gear ratioof the transmission gearings 5 becomes 1. Of course, the othertransistor 88 is kept non-conductive in this case, so that the currentthrough the electromagnetic coil 91 is cut off.

The fundamental, speed change operation of the transmission gearmechanism is thus realized. As a practical matter, however, when therevolutional speed N of the driving axle 6 rises and falls in thevicinity of the revolutional speed NP corresponding to the intersectionpoint between the curves 1 and II in FIG. 2, the speed change operationbecomes unstable. In order to stabilize it, the present invention feedsback the collector voltage of the transistor 80 of the Schmitt circuit78 to the base of the transistor 79 through the resistor 84, therebyproviding the hysteresis characteristic. More specifically, the Schmittcircuit 78 has the transistor 80 turned on in the rising process of therevolutional speed of the driving axle 6, so that no voltage is appliedto the base of the transistor 79 through the feedback resistor 84. TheSchmitt circuit 78 therefore operates so that the revolutional speed Nof the driving axle 6 may become a revolutional speed NP slightly higherthan the set revolutional speed NP indicated in FIG. 2. Then, the changegearratio of the transmission gear mechanism 5 is changed-over to l atthe revolutional speed NP, and the output characteristic of the drivingaxle 6 shifts from curve II to curve I. Herein, since the transistor 80of the Schmitt circuit 78 is kept off in the falling process of therevolutional speed N at curve I, the collector voltage of the transistor80 is kept applied through the feedback resistor 84 to the base of thetransistor 79. As a result, the voltage corresponding to therevolutional speed N of the driving axle 6 as is applied from thecapacitor 75 returns the Schmit circuit 78 at a speed NP" which is lowerthan the set speed NP. The hysteresis width (NP' NP") may be optionallyset by the value of the feedback resistor 84.

While the foregoing embodiments have been described only of the casewhere the change gear ratios of the transmission gear mechanism are land 2, it is apparent that the invention is applicable to cases of otherchange gear ratios. In addition, it is a matter of course that thenumber of stages of the speed change operation may be increased to 3, 4or more stages.

We claim:

I. A driving apparatus for a motor vehicle, comprising a DC powersource, a voltage controller, and an electric driving motor, saidelectric driving motor being powered by said DC power source connectedthereto via said voltage controller, a transmission gear mechanismhaving a variable gear ratio and having an input shaft coupled to saiddriving motor and an output shaft,

driving axle means coupled to said output shaft of said transmissiongear mechanism for driving at least one wheel of the vehicle, detectormeans for detecting the speed of revolution of said driving axle meansand forproviding an output indicative thereof, and speed changing meansresponsive to the output of said detecting means for altering the gearratio of said transmission gear mechanism, said speed changing meansproviding an increased speed reduction gear ratio of said transmissiongear mechanism when the output of said speed detector means is less thanan output corresponding to an output in a predetermined region of theintersection between two predetermined different revolutional speed vs.torque characteristic curves of said driving axle means.

2. A driving apparatus for a motor vehicle according to claim 1, whereinsaid speed changing means includes an electrical level discriminatorcircuit for generating a signal for changing the gear ratio of thetransmission gear mechanism when the ouput of said speed detecting meansexceeds the output corresponding to the predetermined region of theintersection point between said two curves.

3. A driving apparatus for a motor vehicle according to claim 1, whereinsaid two different revolutional speed vs. torque characteristic curvesof said driving axle means correspond to the curves for two differentgear ratios of said transmission gear mechanism.

4. A driving apparatus for a motor vehicle according to claim 1,whereing said speed changing means includes circuit means for providinga first signal when the output of said speed detector means is less thanthe output corresponding to said predetermined region of theintersection point and for providing a second signal when the output ofsaid speed detector means exceeds the output corresponding to saidpredetermined region of the inersection point.

5 A driving apparatus for a motor vehicle according to claim 4, whereinsaid first signal of said circuit means is applied to a firstelectro-magnetic coil for providing said increased speed reduction gearratio and said second signal is applied to a second electromagnetic coilfor providing another gear ratio.

6. A driving apparatus for a motor vehicle according to claim 4, whereinsaid circuit means includes a Schmitt circuit having a predeterminedoutput applied thereto corresponding to the output of the speed detectormeans at the intersection point of said two curves. =l l l l l

1. A driving apparatus for a motor vehicle, comprising a DC powersource, a voltage controller, and an electric driving motor, saidelectric driving motor being powered by said DC power source connectedthereto via said voltage controller, a transmission gear mechanismhaving a variable gear ratio and having an input shaft coupled to saiddriving motor and an output shaft, driving axle means coupled to saidoutput shaft of said transmission gear mechanism for driving at leastone wheel of the vehicle, detector means for detecting the speed ofrevolution of said driving axle means and for providing an outputindicative thereof, and speed changing means responsive to the output ofsaid detecting means for altering the gear ratio of said transmissiongear mechanism, said speed changing means providing an increased speedreduction gear ratio of said transmission gear mechanism when the outputof said speed detector means is less than an output corresponding to anoutput in a predetermined region of the intersection between twopredetermined different revolutional speed vs. torque characteristiccurves of said driving axle means.
 2. A driving apparatus for a motorvehicle according to claim 1, wherein said speed changing means includesan electrical level discriminator circuit for generating a signal forchanging the gear ratio of the transmission gear mechanism when theouput of said speed detecting means exceeds the output corresponding tothe predetermined region of the intersection point between said twocurves.
 3. A driving apparatus for a motor vehicle according to claim 1,wherein said two different revolutional speed vs. torque characteristiccurves of said driving axle means correspond to the curves for twodifferent gear ratios of said transmission gear mechanism.
 4. A drivingapparatus for a motor vehicle according to claim 1, whereing said speedchanging means includes circuit means for providing a first signal whenthe output of said speed detector means is less than the outputcorresponding to said predetermined region of the intersection point andfor providing a second signal when the output of said speed detectormeans exceeds the output corresponding to said predetermined region ofthe inersection point.
 5. A driving apparatus for a motor vehicleaccording to claim 4, wherein said first signal of said circuit means isapplied to a first electro-magnetic coil for providing said increasedspeed reduction gear ratio and said second signal is applied to a secondelectromagnetic coil for providing another gear ratio.
 6. A drivingapparatus for a motor vehicle according to claim 4, wherein said circuitmeans includes a Schmitt circuit having a predetermined output appliedthereto corresponding to the output of the speed detector means at theintersection point of said two curves.